The present invention relates generally to methods for extrusion coating and for extrusion coating and laminating flexible tubular-shaped articles, to coated tubular-shaped articles and to flexible, multilayer, articles of flat or tubular shape.
In many applications utilizing flexible plastic articles, it is important that the articles be exceptionally strong and/or exhibit superior barrier properties. Flexible plastic bags, for example, are commonly made with walls formed of a plastic fabric, such as a woven plastic fabric, to which a thin film or coating of solid plastic is applied. The fabric layer provides the bag with strength and the ability to resist breaking or tearing, and the solid coating functions as a barrier layer to prevent leakage. Examples of articles which require superior strength and/or barrier properties include flexible containers of many types, for example, bags such as bulk, blasting, cement, feed and containment bags; bale wraps, pallet or unitizing wraps; and conveying, capture and/or transport tubing.
Many articles currently on the market are not fully satisfactory. For one thing, most plastic-coated fabrics are simply not fully effective in retaining many fine powders or gel-like materials without some leakage, particularly for extended periods of time. Accordingly, for many applications, it is necessary to provide the article with a separate inner liner to more reliably retain the contents of the article.
In addition, the solid plastic coating is sometimes applied to the outer surface of the article. In such articles, the thin, fragile coating is exposed to the outside environment and is highly susceptible to being punctured, abraded, or otherwise damaged such that its effectiveness as a barrier is lessened or destroyed.
To help protect the fragile coating, the solid plastic layer is often applied to the inner surface of the article. With such a construction, however, the coating is still exposed to the contents of the article and can be easily damaged thereby.
Flexible plastic articles are frequently manufactured from coated, flexible, plastic fabric tubing. To coat the tubing, the tubing is first collapsed upon itself to provide two generally flattened, opposed sides that are integrally and continuously connected together. A thin plastic coating is then applied to the outer surfaces of the flattened sides in a continuous extrusion-coating process to provide a coated, tubular-shaped article having a fabric inner layer and a thin, solid, plastic, outer coating.
This process is not fully satisfactory. Initially, the process tends to produce coatings which have weakened areas, particularly along the edges of the flattened sides where the coatings are joined together. Such weakened areas provide locations where the integrity of the coating is most likely to be broken and where leakage can occur. Secondly, the process often produces jagged portions of laminate which extend outwardly from the edges of the flattened, tubular-shaped article and which are usually removed by trimming. This trimming step not only increases manufacturing costs and results in a less attractive product, but is also extremely difficult, and if not done correctly, can nick and ruin the edge of the flattened tube. Thirdly, to properly apply the coating, a substantial force must be applied across the entire width of the flattened tube, including at the creased edges thereof. Such force can structurally damage the fabric tube, particularly at the creased edges. Fourthly, relatively heavy fabrics are not easily handled by conventional extrusion-laminating equipment, and, during coating, the edges of the flattened tube tend to move slightly or roll over leaving uncoated areas which will leak.
In addition, to simplify manufacturing procedures, the solid coating is usually applied to the outer surface of the fabric tube; and it is necessary to turn the cut lengths of tubing inside out to place the coating on the inner surface. This step can easily damage the fragile coating.